Steps to reproduce

1. Login as Admin
2. Go to Administer Wiki > Look & Feel > Themes
3. Click 'Customize' to edit the theme (tested on default Iceberg theme)
4. On "Advanced" section, set the @xwiki-border-color to purple by adding:

   @xwiki-border-color: #ff00ff;

1. Save and open the Main.WebHome page in View Mode
2. Observe the Main content area

Expected results

The Main content area has a purple border.

Actual results

No border is present around the Main Content area.

The issue could not be reproduced on XWiki 15.9, 15.5.4 or 14.10.20.